Implement move squares highlighting

Issue #3

frontend/main.js

@@ -7,6 +7,15 @@ function get_json(url) {
   return JSON.parse(request.responseText);
 }
 
+function post_json(url, json) {
+  var request = new XMLHttpRequest();
+  request.open("post", url, false);
+  request.setRequestHeader("Content-Type", "application/json;charset=UTF-8");
+  request.send(JSON.stringify(json));
+  if (request.status != 200) return null;
+  return JSON.parse(request.responseText);
+}
+
 class Backend {
   constructor() {
     this.config = new Configuration(
@@ -20,8 +29,7 @@ class Backend {
     this.config.makeMove(source, target);
   }
   getAvailableMoves(position) {
-    position;
-    return [];
+    return post_json("http://localhost:3000/get_moves/", position);
   }
 }
 
@@ -46,6 +54,7 @@ class Chess {
     let validMoves = this.backend.getAvailableMoves(source);
     if (validMoves === null) return;
     this.activeSquares.selectSquare(this.canvas, source);
+    this.activeSquares.setMoveSquares(this.canvas, validMoves);
     this.moveSource = source;
   }
   finalizeMove(target) {
@@ -53,6 +62,7 @@ class Chess {
       this.backend.makeMove(this.moveSource, target);
     }
     this.activeSquares.unselectSquare(this.canvas);
+    this.activeSquares.unsetMoveSquares(this.canvas);
     this.syncBackend();
     this.moveSource = null;
   }